The present invention relates to a method for determining a rotor frequency and/or a rotor angle of a rotor of a reluctance machine. The present invention further relates to a control device for a converter of a reluctance machine. Finally, the present invention relates to a drive assembly.
The present invention is concerned with reluctance machines, in particular synchronous reluctance machines without an amortisseur. Such synchronous reluctance machines without amortisseur, when operated in a field-oriented manner with a frequency converter, exhibit a very good level of efficiency at the same time as good dynamic performance, in particular even under partial load. In contrast with asynchronous machines, there is no rotor time constant to “disrupt” the buildup of dynamic flux, and therefore the synchronous reluctance machine has the advantage of running with utmost energy efficiency under partial load with reduced flux, while at the same time magnetizing quickly in response to demand for load moment. The advantage in respect of the efficiency level is achieved by the reluctance machine primarily due to the elimination of the rotor losses, since the rotor rotates in a steadily synchronous manner with the stator rotating field and does not have any windings. In comparison with a permanently excited synchronous machine, the main benefits are the significantly lower manufacturing costs of the machine and the potential cost savings in respect of the converter protection.
Control devices, as featured in powerful signal processors for example, are now able to model the non-linearities that are characteristic of the reluctance machine and to manage the associated resource-intensive algorithms. Not least for this reason is there now interest in industry for utilizing the advantages of the synchronous reluctance machine in suitable applications. One of the preferred fields of application is drives for pumps or ventilators, for example. These are usually operated as variable-speed drives and have extended running times, resulting in high potential for energy saving. For reasons of cost, such reluctance machines are in most cases configured without a sensor which can measure the rotational speed and/or commutation position. In the case of ventilators in particular, it is necessary to connect to the rotating reluctance machine, quasi “pick up” the drive, and return to the specified delivery rate. When connecting to the turning or rotating synchronous reluctance machine, the converter must be running at the correct rotational speed and phase in relation to the rotor position. Unlike the permanently excited synchronous machine, the rotor consists only of iron (and air), and therefore the usual evaluation of rotational speed and phase by means of measuring the electromotive force is not possible.
Various methods exist for connecting to rotating asynchronous machines and permanently excited synchronous machines without rotational speed and position sensors. In the case of asynchronous machines, it is sufficient to identify the rotational speed, since the phase is not a machine parameter. For this purpose, for example, a current can be impressed at a specified varying search frequency. The search usually starts at a maximum frequency and moves towards zero. The rotational speed to be identified for the machine lies at the point of maximum voltage. It is also possible to use a complete machine model, e.g. in the form of an observer. These have a large coverage area and settle at the machine frequency when supplied with the real machine voltage. In the case of permanently excited synchronous machines, the rotational speed and the phase must be determined. For this purpose, it is possible to measure the electromotive force, which reflects the rotational speed and the phases of the rotor. A test pulse method can also be used, in which a sequence of zero vectors and pulse blocks are generated and the induced pulse currents are evaluated.
As an alternative to the methods described above, another possible method for a synchronous reluctance machine also uses observers, as in the case of an asynchronous machine, which have a large coverage area and settle in phase and angle from a specified start value.
In this regard, the publication “Sensorless Control of Reluctance Machines at Arbitrary Operating Conditions Including Standstill”, M. Schroedel et al., IEEE Transactions on Power Electronics, Vol. 9, No. 2, 1994 describes a method for determining a rotor angle of a reluctance machine. Various voltage vectors can be provided for this purpose, in order to determine the reactances in the d-axis and q-axis.